1. Field of the Invention
The present invention relates to a silicon seed crystal and a method for manufacturing a silicon single crystal. More particularly, it relates to a silicon seed crystal to be preferably used for pulling up of a large-diameter and high-weight silicon single crystal ingot by a Czochralski Method (CZ method), and a method for manufacturing a silicon single crystal using the same.
2. Description of the Related Art
In the manufacturing of a silicon single crystal by the CZ method, a seed crystal made of a silicon -single crystal is brought in contact with a raw material silicon melt, and then slowly pulled up while being rotated, thereby growing a single crystal ingot.
With the CZ method, when a seed crystal is brought in contact with the raw material silicon melt, slip dislocations occur in high density in the seed crystal by thermal shock.
Conventionally, the following method has been adopted. In order to eliminate the dislocations which propagate from the slip dislocations to a single crystal to be grown, a necking step of forming a neck reduced in diameter to about 3 mm is performed, and then, the single crystal is increased in size to a prescribed diameter. As a result, a dislocation-free silicon single crystal is pulled up. Such necking from the seed crystal is widely known as the Dash Necking method, which has been a common method in the manufacturing of a silicon single crystal by the CZ method.
However, with the Dash Necking method, the strength is insufficient for supporting the single crystal ingot increased in weight with a recent trend of an increase in diameter of a silicon single crystal. This might have caused a serious accident such as the dropping of the single crystal ingot due to the fracture of the neck in the pulling-up process of the single crystal ingot.
Under such circumstances, various studies have been made to form a thick neck by the use of a large-diameter seed crystal.
However, the following problem has occurred. The larger the diameter of the seed crystal is, the larger the heat capacitance in the tip portion of the seed crystal in contact with a raw material silicon melt becomes. For this reason, at the instant when the seed crystal comes in contact with the raw material silicon melt, a temperature difference is suddenly caused in the seed crystal. This causes slip dislocations to become more likely to occur in high density.
In response to such a problem, there are disclosed a kind of the method in which a seed crystal is thermally insulated or heated on a raw material silicon melt, thereby to reduce the initial dislocation density, or the shape of the seed crystal is inverted, thereby to reduce the thermal shock upon contact with the melt such as disclosed in JP-A-8-319192 or JP-A-10-203898.
Whereas, there is disclosed a method in which a seed crystal is doped with boron (B) or germanium (Ge) in a high concentration, thereby to reduce the slip dislocations occurring upon contact of the seed crystal with the melt such as disclosed in JP-A-4-139092 or JP-A-2001-199789.
However, with such a method of thermal insulation or heating of a seed crystal as disclosed in JP 8-319192, or such a method using a seed crystal of a specific shape as disclosed in JP 10-203898, there is a limitation on the thickness of the neck. Accordingly, the method is not satisfactorily adaptable to a single crystal ingot which is large in diameter and high in weight, and it cannot necessarily also be said that the dislocation-free rate is enough.
Whereas, with such a method using a B-doped seed crystal as disclosed in JP 4-139092, B which is an impurity for changing the resistivity of the crystal is added. For this reason, unfavorably, the method cannot be used for purposes other than the manufacturing of a low resistivity crystal.
Further, such a method using a Ge-doped seed crystal as disclosed in JP2001-199789 is suitable for the manufacturing of a Ge-doped silicon single crystal or a mixed crystal of Si/Ge. However, it cannot be utilized for the manufacturing of a high purity silicon single crystal for which the mixing of Ge is not preferable.
Therefore, there has been a demand for a method capable of growing a thick and short neck simply without using the seed crystal of a specific shape or doped with a specific high-concentration impurity as described above.
Under such circumstances, the present inventors have conducted a study on the following method. For the growth of a silicon single crystal ingot in conformity with the trend toward a larger diameter and a higher weight, a thick and short neck capable of supporting the whole weight of the single crystal by only a seed crystal and a neck grown from the seed crystal is grown without depending upon a means for thermally insulating or heating the seed crystal, or without using a seed crystal doped with a high-concentration impurity leading to the mixing of an excess impurity, or a seed crystal of a specific shape not capable of being used repeatedly. Thus, they completed the invention.